Liquid developer for xerography



April 7, 1970 K. M. OLIPHANT 3,505,231

LIQUID DEVELOPER FOR XEROGRAPHY Original Filed Feb. 23, 1962 Fl-TF5 5- United States Patent Ofi ice 3,505,231 Patented Apr. 7, 1970 US. Cl. 252--62.1 2 Claims ABSTRACT OF THE DISCLOSURE :Electrostatic images are developed by a liquid toner consisting essentially of two liquids, one being a toner phase liquid dissolved and the other which is a carrier phase liquid.

This application is a continuation of application Ser. No. 174,990, filed Feb. 23, 1962, now abandoned.

This invention relates to liquid developers for electrostatic images with particular reference to the development of electrostatic images on plates or papers and other backings of the kind used in electrophotographic processes in which electrostatic latent images are produced on charged plates by purely electrical or by photographic means. The invention also has reference to the development of visible records or reproductions or copies or patterns on insulating or conducting surfaces wherein a permanent or a fleeting charge pattern has been established electrically such as by electrical signals or photographically such as by optical or X-ray means.

The invention also applies to those processes wherein an insulating layer or a photo-insulating layer is charged electrostatically and then exposed to form a latent image which is developed by a developer to give an image which may then be transferred to a sheet of transfer paper or other material. The invention has for its purpose to afford a developer which has a liquid for its carrier phase and also contains as a developer or toner phase another li uid which is deposited selectively according to the image charge and modifies the surface of the developed sheet in the image areas on a long term basis or a short term basis according to the toner liquid used.

The invention has for a further purpose to provide a liquid developer in which the toner material is a liquid which temporarily modifies the surface on which it is deposited or acts as a shield to protect the underlying sheet against subsequent charging or alternatively acts to protect the underlying sheet against loss of charge or in the case of a strongly insulating toner liquid to enhance the charge in the desired image areas.

A typical electrostatic printing process in which the invention may be used can include the steps of producing an overall blanket electric charge on the surface of a photoconductive material such as selenium, anthracene, arsenic, cadmium sulphide, or zinc oxide dispersed in an insulating binder or produced by vacuum evaporation on a suitable surface. A light image or X-ray image is focussed or shadowed on the surface as the case may be so as to discharge the charged surface in the light-struck or X-ray irradiated areas, leaving the charge on the masked areas and thereby forming a latent electrostatic image.

The electrostatic image is made visible by subjecting the surface to a body of liquid in which is suspended a material which can migrate under the influence of the electric field associated with the latent electrostatic image, so as to deposit on the image areas where said suspended material is of opposite polarity to that of the image and in the non-image areas Where said suspended material is of the same polarity as the charge on the image areas. The present invention relates to all of those cases where the suspended material or the toner as it may be called is a liquid and not a solid particle or particles as heretofore.

Developers previously used for the purpose of rendering electrostatic images visible have included particulate dry material or powders, for example developers known as cascade developers in which the carrier phase is a solid particle or series of particles on which are carried particles or powders of substantially smaller size being held in position by and charged by triboelectric forces. Another type of developer is known as a powder cloud developer in which fine powders are particles are suspended in air and are directed at the surface of the sheet to be developed. Another variation of the cascade developer is known as the magnetic brush developer in which the solid carrier particles are magnetic and are maintained in brush form by magnetic forces. Still another form of developer now considered to be superior to any of the others is embodied in systems which have a liquid for the carrier phase and solid particles as the suspended or dispersed phase. All of these developers have for their purpose the deposition of solid particles in pattern form according to the attraction of the image on the electrostatic sheet. In another form of powder cloud or aerosol development the suspended matter in the air or gases comprises liquid material which however is deposited on the image in the form of droplets which could be called a mist but which have the disadvantage that after deposition of the discrete droplets said droplets spread out and result in a relatively diffuse image as said spreading bears no relation to the image but only to the absorption of the sheet or surface.

Accordingly one object of the present invention is to atford a developer in which the solid particles, which of course limit resolution of the image, are replaced by liquid molecules which are not merely suspended in the carrier gas or liquid but comprise another liquid which is in solution therein and deposits selectively according to the charge or electric field associated with the charge on the electrostatic sheet.

In the known art of developing xerographic images a liquid developer is used which comprises a particulate material suspended in a carrier liquid of high electrical volume resistivity and the particulate material is deposited out of the carrier liquid as a result of the attraction of a latent electrostatic image on an electrostatic printing sheet or under the influence of the electric field associated with the image charge. In the known art therefore a liquid carrier phase supports or contains a solid or particulate developer or toner phase. In the present invention a basically new form of development is possible in that the liquid carrier phase supports or contains a liquid developer phase or toner and the invention relates to the use of two or more liquids one of which acts as the carrier liquid and carries or contains the other, the carrier liquid being characterised by having the necessary high electrical resistivity to prevent the destruction of the charge associated with the latent electrostatic image on the electrophotographic surface or insulator surface during development and the liquid developer phase or toner has the characteristic that it deposits on the surface being developed selectively so as to cause a temporary or long term change in the nature of the surface being developed. In one modification, for example, the invention can be applied by using a liquid carrier phase or vehicle having an electrical volume resistivity in excess of 10 ohm-centimeter and a dielectric constant less than 3 and suspending in this liquid a small quantity of a very strong solvent such as toluene or methyl isobutyl ketone each of which has a lower resistivity and higher dielectric constant than the carrier liquid.

In further explanation of this invention, it has been found that when a strong solvent is present in small quantities in a suitable carrier liquid this strong solvent will deposit preferentially on to the charged image areas and the said solvent will then attack the binder in .the photoconductive coating selectively so as to produce a permanent image in the surface of the sheet. It has also been found that the solvent used can contain a dye and so colour the areas on which the solvent is selectively deposited. By such means it is then possible to remove the binder from the areas where deposition of the solvent has taken place and the latent image has been developed or, alternatively, to modify the surface on which the solvent material has been deposited.

Although we do not wish to be bound to the following theory this theory will help in the better understanding of the invention. The molecular or colloidal material of one disperse phase suspended in another dispersion medium phase can be considered to move selectively under the influence of an electric field in accordance with the Helmholtz or the Pellat modified equation which is as follows:

z.X 41rn where V is the velocity of migration of the colloidal micelle or molecule, z is the electrokinetic potential (potential drop across the electrical double layer), X is the applied electric field, and n is the internal friction of the medium (viscosity). According to Pellat the effect of the dielectric constant of the external medium can be taken into account as follows:

' 41m where D is the dielectric constant of the surrounding liquid.

From this theory it Will be appreciated that the two liquids of the present case will separate according to the difference in their dielectric constants, and to the electric field, and to the magnitude of the electrokinetic potential between the dispersed phase and the dispersion medium, and in inverse proportion to their viscosities. It is further apparent that the viscosities of the liquids will have a bearing on the mode of deposition of the toner liquid in the image areas, other factors constant.

The invention consists therefore essentially in the separation of the dispersed liquid from its liquid environment under the influence of the electric field associated with the image and that this separation cl arly depends on the dielectric constants of the two liquids or the difference between them, the intensity of the electric field to which they are conjointly subjected, and the difference in the viscosity of the toner compared with the carrier.

The invention obviously has a very wide application and a particular use for the invention can be in the production of reversed xerographic images because the first development with a liquid phase developer is capable of removing or modifying the photoconductor surface in such a way that it will not subsequently take a charge and therefore the image can subsequently be developed in reverse with a normal type of pigmented developer. In another application of the invention the surface to be developed can in the first instance have a colour applied to the whole surface by any means such as electrophotographic or electrostatic printing or by mechanical printing or by dyeing and subsequently the surface can be charged and exposed and developed with a liquid phase developer so as to deposit a liquid in image or pattern form said liquid being a solvent for the colour applied in the first instance so as to remove or modify the colour in accordance with the liquid solvent of the developed image or pattern.

Obviously as it possible to deposit a liquid phase or toner preferentially on to a surface arranged to receive it a very wide application is achieved as the deposited liquid can operate as a solvent to remove the base on which it is deposited or it can act chemically to change the base such as by changing its colour or it can affect the electrical properties of the base in such a way that re-charging of these areas or other effects are not possible and a further possibility is that a dye or other colouring agent can be introduced into the liquid de- Veloper phase or toner and deposited with the liquid in pattern form having been carried over or deposited with the liquid in solution form. However it will be realised that the normal processes of development of an electrostatic image do not apply but are in fact superseded, as according to the present invention the liqui d medium which is deposited can act preferentially on the base carrying the image to be developed and can modify or colour the base if desired without effecting an image deposition thereon.

In further explanation of this invention, it has been found where polar solvents are used for the liquid toner phase or part of the liquid toner a process of precipitation may ensue and this may have to be repeated several times or a variety of non-polar solvents may have to be used to control precipitation so as to eliminate any excess of polar solvent the presence of which would act to lower the volume resistivity of the carrier liquid to the point where development would cease to function due to destruction of the latent image before completion of de velopment.

The novel features of the present invention will be understood in more detail when considered in connection with the accompanying drawing in which similar reference characters represent similar parts and in which:

FIG. 1 shows a schematic diagram of a sheet covered with an insulator or photoinsulator or a photoconductive or a semi-conductive coating or layer, on which there exists an electrostatic pattern or image,

FIG. 2 shows this sheet after it is imersed in or has been subjected to a body of liquid in which there is contained a liquid developer phase,

FIG. 3 shows the sheet after the liquid developer phase or toner liquid has been selectively concentrated in the image or pattern areas, and

FIG. 4 shows the sheet after the selectively deposited or concentrated liquid developer phase has been permitted to modify the surface of the sheet. These figures of course refer only to one embodiment of the invention and do not restrict the invention.

In FIG. 1 a sheet of metal or paper or wood or other material is coated with an insulator material such as a resin film or a photoinsulator such as selenium or a layer of a photoconductor such as zinc oxide embedded in an insulating film for example an alkyd film or an epoxy resin film or the whole sheet may itself merely comprise a sheet of insulating material such as a polyester plastic sheet. The sheet is designated 1 and the backing thereto is designated 2, and the charge pattern thereon is designated 3. FIGURE 2 shows the sheet I backed by the backing material 2 and having on the surface thereon the charge pattern 3 in the areas 4 and 5 of the pattern or image to be reproduced or recorded and subjected to a body of liquid of high volume resistivity 6 containing suspended therein a toner or development liquid phase 7 said toner or liquid being freely available to move in the body of the carrier liquid under the influence of the electric field associated with the image or pattern on the sheet 1 and comprising molecules or colloidal material or globules suspended or in solution or present as a colloid or as micelles or encapsulated in resin or as swollen precipitates or as a supersaturated solution which could be precipitated by the charges 3 in the areas 4 and 5.

FIGURE 3 shows the sheet I backed by the backing 2 having on the surface of the sheet 1 in the areas 4 and 5 the electric charges 3 surmounted by the attracted liquid toner or developer phase deposits 8 and 9 which have been selectively deposited in the image areas 4 and 5 by the electric field associated with the charges 3.

FIGURE 4 shows the sheet 1 backed by the backing 2 having on the surface of the sheet modified material 10 and 11 in the image areas 4 and 5 comprising material formed by the interaction of the developer toner liquid with the sheet material to form a new material or the original sheet material so modified by the image liquid as to change its electrical or other physical properties such as conductivity or dielectric constant or magnetic properties or colour.

To show the way in which the invention can be carried out or carried into effect the following examples are given but it will be apparent that the invention is not limited to these.

EXAMPLE 1 A liquid developer was prepared by dissolving ogether the following liquids which will separate when subjected to an electric field:

Parts by volume n-Heptane 100 Carbon tetrachloride 10 In this case the liquid carrier phase was the n-heptane and the developer phase was the carbon tetrachloride.

A sheet of electro-photographic paper was charged and exposed or a sheet of insulating plastic was charged in pattern form and then developed in the above liquid developer for 30 seconds and then dried off in a warm air blast. The electrophotographic sheet was then exposed to light from a 160 watt lamp at a distance of two feet for 10 seconds to disperse the residual picture charge. The sheet of insulating plastic would not of course be required to be so illuminated. The sheet was then recharged and developed with a black developer which developed all of those areas which had not been developed with the first liquid toner but had been rendered relatively conductive by residual carbon tetrachloride or had surface properties temporarily altered in these areas.

EXAMPLE 2 A liquid developer was prepared by dissolving together the following liquids:

Parts by volume cyclohexane 100 Oleic acid 1 A similar procedure was adopted to that given in Example 1 in order to demonstrate the relative conductivity of the areas developed with the oleic acid which in this case constituted the developer phase.

EXAMPLE 3 A liquid developer was prepared by dissolving together the following liquids:

Parts by volume n-Heptane 100 Xylol 10 In this case the liquid carrier phase was the n-heptane and the xylol was the developer phase. The xylol will soften an alkyd resin film.

EXAMPLE 4 A liquid developer was prepared by dissolving together the following liquids:

Parts by volume n-Heptane 100 cyclohexanone 10 In this case the developer phase was the cyclohexanone which is not only of lower resistivity but it is definitely a polar liquid.

6 EXAMPLE 5 A liquid developer was prepared as follows:

Carrier phase: Parts by volume Shell Solvent X55 Methyl alcohol 5 The Shell solvent is an aliphatic hydrocarbon solvent known under the trademark Shell X55. The methyl alcohol can act as a solvent for a binder or base of an electrophotographic sheet.

EXAMPLE 6 Parts by volume Carrier liquid phase: n-heptane 100 Developer phase: cyclohexane 10 Containing dye waxoline blue 10 In this case the cyclohexane was dyed with the oilsoluble dye I.C.I. Waxoline blue which enabled a visible image to be developed on the photoconductive or insulating surface.

EXAMPLE 7 A liquid developer of the following composition was prepared:

cc. Iso-octane 100 n-Heptane 10 This was used to develop an electrostatic image on a silicone-resin-bound electrophotographic sheet in which case the n-heptane selectively dissolved the silicone resin in the image areas. The silicone resin which carried the photoconductor medium such as zinc oxide could of course have incorporated in it the dye Rose Bengal or Erythrosin or Methyl blue or the like to give a coloured photoconductor surface which was then removed selectively by the n-heptane to leave a coloured image where the resin was not removed or in other Words a reverse image. Similarly a coloured photoconductor could be used such as cadmium oxide, cadmium sulphide, lead monoxide, selenium, or alternatively fluorescent pigments such as Lumogen red or Lumogen green in which case again the removal of the resin selectively according to the original image was used to obtain the required coloured reproduction, or pattern.

EXAMPLE 8 A conductive liquid developer was prepared as follows:

Carrier phase: trichlorotrifiuorethane100 cc.

Developer phase: 1% solution of ferric chloride (by weight) in an aliphatic hydrocarbon solvent (such as that known under the trademark Shell Solvent This produced a relatively conductive visible image. If the carrier phase or the developer phase was a solvent for the photoconductive surface and only softened but did not completely remove the surface in the image areas or background, as the case might be, the ferric chloride was retained in the photoconductive layer after being deposited in pattern form.

EXAMPLE 9 In the liquid developer of Example 1, perchlorethylene was substituted for the carbon tetrachloride.

EXAMPLE 10 In the formula of Example 2 lauric acid was substituted for the oleic acid liquid developer phase.

EXAMPLE 11 In the liquid developer of Example 3 the xylol was replaced by cumene.

EXAMPLE 12 In the formula of Example 4, the cyclohexane could be replaced by methyl isobutyl ketone.

7 EXAMPLE 13 In Example the methyl alcohol was replaced by allyl or butyl or iso-propyl alcohol.

It will be seen from these examples that the invention resides in the production of a liquid developer for electrostatic images characterized in that the carrier liquid is an insulating liquid which has a volume resistivity sufficiently high to prevent destruction of the electrostatic field of the image charge and the developer phase is a liquid which is dissolved or dispersed or suspended or emulsified in the carrier liquid in such proportions as not to impair the resistivity of the carrier liquid or the total developer and Which can be deposited out of the carrier liquid by the electric fields associated with the latent electrostatic image so as to change or colour the surface of the sheet on which it deposits such sheet having electrophotographic properties or such sheet merely being an insulating sheet or plastic film on which there has been produced a charge image by purely electrical means for instance by electric signals.

Amongst the many uses to which this invention can be put is the application of the invention to the processes of lithography for it will be appreciated that in the areas of the electrostatic sheet in which the properties can be changed this change may take the form of increase or decrease in the hydrophobic or hydrophilic properties of the surface and in this way the sheet may be converted to use as a lithographic master. Whereas generally the methods of placing images on lithographic plates are cumbersome and expensive the present invention provides for an easy and low cost method of rendering the sheet suitable for lithography according to a pattern or image which is very quickly photographed or placed thereon.

Another use for our invention is to produce printing blocks as it will be appreciated that the sheet or plate which is developed by this process can be quickly and easily converted in its surface properties so as to produce acid resistance or resistance to other solvents in the image areas.

Another use is for the production of printed clrcults.

I claim:

1. In a method of developing a latent electrostatic image on an insulator sheet presenting an electrically charged surface comprising, contacting the charged surface with a liquid developer containing a coloring agent and comprising a carrier liquid consisting of a first electrically insulating liquid having an electrical volume resistivity in excess of 10 ohm centimeter and a dielectric constant less than 3, and a toner phase liquid dissolved in the first liquid and which is attracted by the electrostatic field presented by the charged surface, said toner phase liquid having a lower volume resistivity and higher dielectric constant than the carrier liquid, the amount of toner liquid being less than the carrier liquid so as not to impair the resistivity of the carrier liquid, and said liquids being selected from the group consisting of:

Carrier liquid: Toner liquid n-Heptane Carbon tetrachloride. Cyclohexane Oleic acid. n-Heptane Cyclohexane.

Trichlorotrifluoroethane References Cited UNITED STATES PATENTS 3,068,115 12/1962 Gundloch 252-621 LEON D. RO'SDOL, Primary Examiner J. D. WELSH, Assistant Examiner US. Cl. X.R. 

